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Conclusion is that ら is the most popular (and sometimes even や), and my very rough guess is that this has connection to the fact that the epenthetic consonant in Japanese is usually a liquid (r) or a glide (y, w) as you can observe in the phonoloical rule applied to vowel ending verbs: mi-u → miru (見る), classically mi-u → miyu (見ゆ), mi-are-ta → mirareta (見られた)

I've been meaning to ask this for some time but what is epenthesis and what does it do?
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FlawJan 6 '12 at 12:37

Also, I think it can also be a repetition of any syllable that a word ends on.
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FlawJan 6 '12 at 14:15

1

@Flaw Epenthesis is a sound that is inserted in order to repair a sequence of sounds that is not allowed in that language, or for some other reason. For example, "tm" is not allowed in "kat-mas-u", so "i" is inserted in between, yielding "katimasu" (勝ちます). In this case, the vowel sequence "iu", although allowed, is called a hiatus and is broken by a consonant. In Japanese, that consonant is "r".
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user458Jan 6 '12 at 17:59

Where can I learn about phonotactic prohibitions? (I would prefer an English-based resource)
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FlawJan 8 '12 at 2:35

I think the wikipedia articles on "epenthesis" may be helpful. As for what consonant clusters are not allowed in Japanese, it is pretty much symple: besides the nasal coda (ん) and gemination (っ), Japanese does not allow consonants in coda position (i.e., after a vowel), and all possible onset (a consonant before a vowel) are those you can find in the hiragana list. As for hiatus (i.e., vowel sequence), if you cannot find the literature by yourself, probably it is difficult.
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user458Jan 8 '12 at 4:24